The present invention concerns brushless DC motors, and especially brushless DC linear motors of the type comprising at least one permanent magnet system and wound stack of iron laminations, the current flow in the constituent windings being reversed or switched on and off by controllable semiconductor elements operating in dependence upon sensed armature position. Linear motors of such type are often employed in relatively high-precision devices, e.g. in typewriters for the horizontal displacement of the printing head relative to a sheet of paper to be imprinted.
When such linear motors are use in positioning systems, the component to be displaced by such system, e.g. the printing head of a typewriter, should reach the desired position as quickly as possible. To this end the linear motor must exert the highest force realizable in order to effect displacement of such component with high acceleration.
At present, positioning systems which implement linear motions are predominantly equipped with rotary motors connected to the linearly displaced component through the intermediary of speed-reducing gearing and a drive belt. Also, linear motors operating according to the principle of step motors or brushless DC motors find use and are the subjects of ongoing investigation (see, e.g., the West German periodical "Feinwerktechnik & Messtechnik" 89 (1981), Vol. 4, pp. 163-167).
The use of rotary motors has the advantage that, due to the provision of the intermediate speed-reducing gearing, high displacement force can be exerted even with motors of relatively low power. Countervailing disadvantages, however, include the considerable cost of the requisite gearing and drive belts and, also, the often undesirable effects of the latter elements upon the dynamic behavior of the resulting positioning system.
As for linear motors, those types operating in the manner of step motors are far from ideal for positioning systems, due to their effect upon the dynamic behavior of the system: these types of motors tend towards armature oscillation and behave not very muscularly when accelerating the component to be displaced.
In contrast, DC linear motors exhibit comparatively appropriate behavior. However, conventional designs for such motors generally require a higher volume of motor material than a comparable rotary motor; i.e., due to the lack of intermediate speed-reducing gearing, these motors must be designed to exert, quite directly, the full magnitude of the intended displacement-effecting force.